1. Field of the Invention
The present invention relates generally to scroll type fluid machines, including a scroll type refrigerant compressor and, more particularly, relates to a method of manufacturing a movable scroll element accommodated in the scroll type machine and the movable scroll element produced by the method.
2. Description of the Related Art
Scroll type fluid machines are known. For example, a scroll type refrigerant compressor can be provided with a stationary scroll element and a movable scroll element implementing an orbiting motion with respect to the stationary scroll element. As shown in FIG. 5, the movable scroll element includes a movable end plate 90 in the shape of a flat disk, a boss portion 91 projecting axially from one of the opposite end faces of the movable end plate 90, and a spiral member 92 projecting axially from the other of the end faces and extending spirally around a given axis vertical to the movable end plate 90. The boss portion 91 of the movable scroll element is provided for receiving a drive force for causing the orbiting motion.
The stationary scroll element is provided with a disk-like stationary end plate and a stationary spiral member axially projecting from one of the end faces of the stationary end plate so as to extend spirally about a given axis vertical to the stationary end plate.
In a scroll type compressor accommodating the above-mentioned stationary and movable scroll elements in a housing assembly, a drive bush (not shown in FIG. 5) is fitted in an inner bore of the boss portion 91 via a bearing member, and the drive bush is driven by a drive pin member eccentrically attached to a rotating drive shaft of the compressor. Thus, the drive bush moves in an orbiting path so as to cause the orbiting motion of the movable scroll element with respect to the stationary scroll element. The movable scroll element is prevented from being rotated about its own axis by a rotation preventing means intervened between the movable end plate 90 and the housing assembly. The movable spiral member 92 of the movable scroll element is in a constant contact engagement with the stationary spiral member of the stationary scroll element so as to define, therebetween, a plurality of compression chambers which are successively moved from the outer circumference portion of the stationary scroll element toward the center of the spiral elements of the stationary and movable scroll elements. During the movement of the plurality of compression chambers toward the center of the stationary and movable spiral members, the volumes of the respective compression chambers are gradually reduced to compress the refrigerant gas within the respective compression chambers.
Conventionally, the movable scroll element of the scroll type fluid machine is produced by a method as set forth below with reference to the illustration of FIG. 6.
As shown in FIG. 6, a workpiece "W" having a substantially cylindrical body 80 and a substantially cylindrical projection 81 projecting from one of the opposite end faces of the cylindrical body 80 is produced by the sand casting method by using a sand mold having a pair of molds, i.e., a first mold 85 for casting the cylindrical body 80, and a second mold 86 for casting the cylindrical projection 81. Namely, a filler such as a molten metal is poured in a cavity provided between the first and second sand molds 85 and 86 to obtain a cast product "W" shown in FIG. 7A. A parting line "PL" of the first and second sand molds 85 and 86 substantially lies in a plane located adjacent to an end face 82a of an end plate forming portion 82 of the cylindrical body 80.
The cast product "W" obtained by the sand mold casting is subsequently subjected to a metal cutting process. Namely, the product "W" is rigidly held by a machine chuck, at an outer circumference of the cylindrical body 80 as shown by arrows C, and is subjected to a cutting process to finish the end face 82a of the end plate forming portion 82 and a part 82b of the outer circumference of the cylindrical body 80 as shown by dotted lines in FIG. 7A. Further, outer faces of the cylindrical projection 81 including a cylindrical outer face 81a, an end face 81b, an inner cylindrical face 81c, and an inner end face 81d are cut as shown by dotted lines in FIG. 7A. The cut product "W" is shown in FIG. 7B. Then, the cut product "W" is further subjected to a cutting process to finish a spiral portion 83 within an inner portion. of the cylindrical body 80, as shown by dotted lines in FIG. 7B. When cutting the spiral portion 83, the product "W" is held by the machine chuck at the cylindrical outer face 81a of the cylindrical projection 81 as shown by arrows "C". Thus, as shown in FIG. 5, a finishing product of the movable scroll element having the movable end plate 90, the boss portion 91, and the movable spiral member 92 is obtained. Finally, a machining cut of a tip seal groove (not shown in FIG. 5) is carried out to obtain a final product of the movable scroll element.
It should be noted that a different type of a movable scroll element for a scroll type compressor is known in which the movable element has pins with which a drive bush of the drive shaft of the compressor is engaged via bearings to cause an orbiting motion of the movable scroll element. The pins of the different type of movable scroll element are provided instead of the boss portion 91 of the movable scroll element of FIG. 5, and the different type of movable scroll element can be produced by the method substantially identical with the method described with reference to FIGS. 6, 7A, and 7B.
Nevertheless, the above-described conventional method of producing the movable scroll element for a scroll type fluid machine adopted a way of cutting overall outer faces of the cast product "W". Thus, a portion of the scroll movable element, i.e., a drive force receiving portion of the movable scroll element must be cut so as to have an increased thickness capable of obtaining a mechanical strength durable for the drive force provided during the operation of the scroll type compressor. Therefore, the scroll type fluid machines including the scroll type refrigerant compressors must include the movable scroll element having a large wall thickness, and accordingly, the size and weight of the compressor become unduly large from the viewpoint that the compressors mounted on vehicles should be light in weight.
When the drive power receiving portion of the movable scroll element consists of the afore-mentioned boss portion 91, the portion 91 receives, via a bearing, a drive power for causing an orbiting motion of the movable scroll element, and accordingly, the boss portion 91 must have a wall thickness sufficient for exhibiting appropriate mechanical strength. Therefore, if the boss portion 91 of the movable scroll element can be reduced in its wall thickness without sacrificing the mechanical strength thereof, the size and weight of the scroll type compressor can be appreciably reduced so as to permit the compressor to be mounted on the vehicles without any difficulty.
The present inventors have conducted analytical research for improving the mechanical strength of a movable scroll element, especially the boss portion thereof while appreciably reducing a wall thickness of the boss portion. As a result, it was found that in the conventional production method of the movable scroll element, an outermost layer, i.e., a black scale of the cast product "W" which permits the product "W" to have a sufficient mechanical strength, must be removed by machining or cutting. Therefore, the mechanical strength of the finished product, i.e., that of the movable scroll element as shown in FIG. 5 is reduced. The reason why the outermost layer or the black scale of the cast product "W" must be removed is that, since the cast product "W" having the cylindrical body 80 and the cylindrical projection 81 is obtained by using the sand molds 85 and 86 having the parting line PL lying in the plane located adjacent to the end face 82a of the cast product "W", it is difficult to obtain a coaxial condition between the center "O1" of the cylindrical body 80 and that "O.sub.2 " of the cylindrical projection 81 of the cast product "W". Thus, the machining process for establishing the coaxial condition was indispensable for obtaining an accurate movable scroll element.